Rudder Cable Systems Used in Modern Sailplanes

On 15/05/2012 23:41, Dan wrote:
On May 15, 6:33 am, wrote:
What would be the consequence of removing the springs from the rudder pedals?

I used to fly competition aerobatics in a Pitts S-1S I built. I
became worried about rudder return springs in that airplane for the
same reasons. Eventually I removed the springs and I actually liked
the feel of the rudder much better.

Dan
ASW 20
WO

Might be fine in your Pitts but my memory is that the ASW20 manual made
a special point about NOT using lower strength rudder springs. I also
remember them as the strongest I've come across in the dozen or so
gliders I've worked on.

GC

On May 15, 10:05*am, GC wrote:
On 15/05/2012 23:41, Dan wrote:

On May 15, 6:33 am, wrote:
What would be the consequence of removing the springs from the rudder pedals?

I used to fly competition aerobatics in a Pitts S-1S I built. *I
became worried about rudder return springs in that airplane for the
same reasons. *Eventually I removed the springs and I actually liked
the feel of the rudder much better.

Dan
ASW 20
WO

Might be fine in your Pitts but my memory is that the ASW20 manual made
a special point about NOT using lower strength rudder springs. *I also
remember them as the strongest I've come across in the dozen or so
gliders I've worked on.

GC

Spring strength may be a requirement to control/dampen flutter...
Control dynamics are not something to play with lighly!

Cheers,


Derek

On May 15, 12:35*am, Ramy wrote:

Checked the rudder cables on my 27 - looking good. But as JJ described,
loosening the tension on one side causes a hard over to the other side!

I do not necessarily agree with JJ's synopsis of the situation. Yes,
on the ground removing the tension of one spring will cause the rudder
to go to the opposite stop. However, in the air there is airflow over
the rudder that would counteract the force of the spring. How much
effect this has depends on the strength of the return spring as well
as other aerodynamic factors. In all of the sailplanes I've flown, the
rudder springs are pretty wimpy in relation to the aerodynamic forces
involved. 14CFR23 and JAR22 dictate that the rudder circuit of light
aircraft is to be designed to react at least 150 lbs per pedal and a
combined force of 300 lbs on the pedal pair unless a lower force can
be rationally justified.

On that basis, it is my conjecture that there is an additional factor
at work in the JS1 incident besides a broken rudder cable. I am
standing by to see if such a factor comes to light.

I would like to hear how glider manufactures defends this design!

As a glider designer, I defend it so:

Cable actuation systems are a simple and effective approach to the set
of problems at hand. They are easy to inspect and service, and
problems are easy to detect. The 1/8" (3mm) cables commonly used have
about a 4x safety factor over typical maximum control forces, so they
will take a lot of abuse before failing. Every experienced A&P and IA
knows to inspect cables in their areas of tightest curvature, and
these inspections bring to light the vast majority of potential
problems long before they become critical. As typically implemented in
sailplane fuselages (including the three I have so far built), the
cable-in-tunnel system has the additionally compelling advantage of
adding increased rudder damping when the pilot applies pressure to
both pedals. This feature has been successfully used to damp incidents
of rudder flutter in quite a number of incidents that might otherwise
have eventually resulted in resonant failure of the aft fuselage.

The one issue I have with typical sailplane rudder cable systems is
that the S-tube on the side of the pedal that allows for pedal
position adjustment can cause a short-radius curvature of the rudder
cable at extremes of pedal deflection. The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point.

As a counterexample rudder actuation system, I submit the Diamant. The
makers went to heroics to reduce rudder actuation friction, using push-
pull tubes in linear roller bearings with many ball bearing pivots and
a rather complicated pedal adjustment system. What they got was rudder
flutter, and they ended up having to incorporate a hydraulic shock
absorber in order to apply damping to the system. So they started with
a complicated system and ended up having to make it more complicated
yet before it was fully functional. Think of all the things they could
have done with their energy had they just used a standard cable system
and moved on.

I wonder how many were killed by this design, giving many unexplained spins into
the ground from higher altitude.

My guess is few to none. Here in the US, crash investigations of light
aircraft, especially those of gliders, do tend to be less systematic
than those of larger aircraft. However, in my experience crash
investigators are fully competent at recognizing the signatures of
wear and fatigue failures in cable-actuated control systems. Where
such signatures are found, they are usually announced prominently in
the accident synopsis.

Thanks, Bob K.

On Tuesday, May 15, 2012 11:50:23 AM UTC-7, Bob Kuykendall wrote:
On May 15, 12:35*am, Ramy wrote:

Checked the rudder cables on my 27 - looking good. But as JJ described,
loosening the tension on one side causes a hard over to the other side!

I do not necessarily agree with JJ's synopsis of the situation. Yes,
on the ground removing the tension of one spring will cause the rudder
to go to the opposite stop. However, in the air there is airflow over
the rudder that would counteract the force of the spring. How much
effect this has depends on the strength of the return spring as well
as other aerodynamic factors. In all of the sailplanes I've flown, the
rudder springs are pretty wimpy in relation to the aerodynamic forces
involved. 14CFR23 and JAR22 dictate that the rudder circuit of light
aircraft is to be designed to react at least 150 lbs per pedal and a
combined force of 300 lbs on the pedal pair unless a lower force can
be rationally justified.

On that basis, it is my conjecture that there is an additional factor
at work in the JS1 incident besides a broken rudder cable. I am
standing by to see if such a factor comes to light.

I would like to hear how glider manufactures defends this design!

As a glider designer, I defend it so:

Cable actuation systems are a simple and effective approach to the set
of problems at hand. They are easy to inspect and service, and
problems are easy to detect. The 1/8" (3mm) cables commonly used have
about a 4x safety factor over typical maximum control forces, so they
will take a lot of abuse before failing. Every experienced A&P and IA
knows to inspect cables in their areas of tightest curvature, and
these inspections bring to light the vast majority of potential
problems long before they become critical. As typically implemented in
sailplane fuselages (including the three I have so far built), the
cable-in-tunnel system has the additionally compelling advantage of
adding increased rudder damping when the pilot applies pressure to
both pedals. This feature has been successfully used to damp incidents
of rudder flutter in quite a number of incidents that might otherwise
have eventually resulted in resonant failure of the aft fuselage.

The one issue I have with typical sailplane rudder cable systems is
that the S-tube on the side of the pedal that allows for pedal
position adjustment can cause a short-radius curvature of the rudder
cable at extremes of pedal deflection. The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point.

As a counterexample rudder actuation system, I submit the Diamant. The
makers went to heroics to reduce rudder actuation friction, using push-
pull tubes in linear roller bearings with many ball bearing pivots and
a rather complicated pedal adjustment system. What they got was rudder
flutter, and they ended up having to incorporate a hydraulic shock
absorber in order to apply damping to the system. So they started with
a complicated system and ended up having to make it more complicated
yet before it was fully functional. Think of all the things they could
have done with their energy had they just used a standard cable system
and moved on.

I wonder how many were killed by this design, giving many unexplained spins into
the ground from higher altitude.

My guess is few to none. Here in the US, crash investigations of light
aircraft, especially those of gliders, do tend to be less systematic
than those of larger aircraft. However, in my experience crash
investigators are fully competent at recognizing the signatures of
wear and fatigue failures in cable-actuated control systems. Where
such signatures are found, they are usually announced prominently in
the accident synopsis.

Thanks, Bob K.

Thanks Bob for your insight and thorough explanation. It makes sense.

Ramy

At 18:05 15 May 2012, Derek Mackie wrote:
On May 15, 10:05=A0am, GC wrote:
On 15/05/2012 23:41, Dan wrote:

On May 15, 6:33 am, wrote:
What would be the consequence of removing the springs from the
rudder
=
pedals?

I used to fly competition aerobatics in a Pitts S-1S I built. =A0I
became worried about rudder return springs in that airplane for the
same reasons. =A0Eventually I removed the springs and I actually
liked
the feel of the rudder much better.

Dan
ASW 20
WO

Might be fine in your Pitts but my memory is that the ASW20 manual made
a special point about NOT using lower strength rudder springs. =A0I
also
remember them as the strongest I've come across in the dozen or so
gliders I've worked on.

GC

Spring strength may be a requirement to control/dampen flutter...
Control dynamics are not something to play with lighly!

Cheers,


Derek

Ah! you have illuminated a point about rudder flutter.
In 1972 the bank bought me a kestrel 19 for an important comp.
I discovered in the precontest flying that it was prone to rudder flutter
well below rough air speed, quickly damped by pushing on both pedals, but
it gave one pause when aiming for the start line.
3 years later I acquired same kestrel in pieces after a syndicate had bent
it. On inspecting the fuselage layup, I found that this
was orthogonal to, not 45 deg to the Cl. Hence the rudder flutter
, fuselage lacked torsional rigidity. The repair was acording to factory
specs, but I did add mass balance to the rudder at the TOP.
Test flights showed the flutter speed had moved up above the
roughair speed; much better.
Now I understand why cables in PE tubing are much safer.
JMF

This may be faulty memory, but I seem to recall that the Open
Cirrus used a pushrod rudder drive with cable drive from pedals
to just behind the seat; early ones suffered from rudder flutter
and a hydraulic damper was fitted to the system. Subsequent
Schemmp-Hirth designs ran the rudder cables all the way to the
rear fuselage, and the guide tubes were slightly waved to
increase friction for rudder damping.

Ah! you have illuminated a point about rudder flutter.
In 1972 the bank bought me a kestrel 19 for an important
comp.
I discovered in the precontest flying that it was prone to rudder
flutte
well below rough air speed, quickly damped by pushing on
both pedals, bu
it gave one pause when aiming for the start line.
3 years later I acquired same kestrel in pieces after a
syndicate had ben
it. On inspecting the fuselage layup, I found that this
was orthogonal to, not 45 deg to the Cl. Hence the rudder
flutter
, fuselage lacked torsional rigidity. The repair was acording to
factor
specs, but I did add mass balance to the rudder at the TOP.
Test flights showed the flutter speed had moved up above the
roughair speed; much better.
Now I understand why cables in PE tubing are much safer.
JM

On 5/15/2012 12:35 AM, Ramy wrote:

When checking the rudder cables for broken strands it is best to
use a soft tissue which will readily snag on the broken strand. Of
course you can use your fingers if you like and if you have any
broken strands they can be easliy seen by the bloody drippings on
the cable.

Checked the rudder cables on my 27 - looking good. But as JJ
described, loosening the tension on one side causes a hard over to
the other side! I would like to hear how glider manufactures defends
this design! I wonder how many were killed by this design, giving
many unexplained spins into the ground from higher altitude. A glider
may still be landable without rudder control, but not with a full
rudder. Couldn't they come up with a design with a more graceful mode
of failure??

Since it's something that gradually goes bad (many flights from the
first strand breaking until cable failure) and is easily checked during
preflight, I don't think it's an issue. If not caught during preflight,
I think it would last till the annual inspection.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On 16/05/2012 15:06, Eric Greenwell wrote:
On 5/15/2012 12:35 AM, Ramy wrote:


...Couldn't they come up with a design with a more graceful mode
of failure??

Since it's something that gradually goes bad (many flights from the
first strand breaking until cable failure) and is easily checked during
preflight, I don't think it's an issue. If not caught during preflight,
I think it would last till the annual inspection.

I'd agree with you except for the JS-1 which began this discussion. I
find it hard to believe (and unacceptable) that a glider which can only
be a year or so old has had a broken rudder cable from normal wear and tear.

Perhaps there was wear against the top of the S-tube - maybe the plastic
insert had migrated down. Otherwise, something failed which isn't
expected to wear or break - a swage slipped, anchor fitting broke - I'll
be interested to read the report.

Also, with great respect to Bob K and his experience, I'm not convinced
airflow will sufficiently straighten a rudder with a broken cable. Even
in the best case there will be residual deflection where the airflow
balances the other spring. This could be quite large.

GC

On May 15, 12:50*pm, Bob Kuykendall wrote:
On May 15, 12:35*am, Ramy wrote:

Checked the rudder cables on my 27 - looking good. But as JJ described,
loosening the tension on one side causes a hard over to the other side!

I do not necessarily agree with JJ's synopsis of the situation. Yes,
on the ground removing the tension of one spring will cause the rudder
to go to the opposite stop. However, in the air there is airflow over
the rudder that would counteract the force of the spring. How much
effect this has depends on the strength of the return spring as well
as other aerodynamic factors. In all of the sailplanes I've flown, the
rudder springs are pretty wimpy in relation to the aerodynamic forces
involved. 14CFR23 and JAR22 dictate that the rudder circuit of light
aircraft is to be designed to react at least 150 lbs per pedal and a
combined force of 300 lbs on the pedal pair unless a lower force can
be rationally justified.

On that basis, it is my conjecture that there is an additional factor
at work in the JS1 incident besides a broken rudder cable. I am
standing by to see if such a factor comes to light.

I would like to hear how glider manufactures defends this design!

As a glider designer, I defend it so:

Cable actuation systems are a simple and effective approach to the set
of problems at hand. They are easy to inspect and service, and
problems are easy to detect. The 1/8" (3mm) cables commonly used have
about a 4x safety factor over typical maximum control forces, so they
will take a lot of abuse before failing. Every experienced A&P and IA
knows to inspect cables in their areas of tightest curvature, and
these inspections bring to light the vast majority of potential
problems long before they become critical. As typically implemented in
sailplane fuselages (including the three I have so far built), the
cable-in-tunnel system has the additionally compelling advantage of
adding increased rudder damping when the pilot applies pressure to
both pedals. This feature has been successfully used to damp incidents
of rudder flutter in quite a number of incidents that might otherwise
have eventually resulted in resonant failure of the aft fuselage.

The one issue I have with typical sailplane rudder cable systems is
that the S-tube on the side of the pedal that allows for pedal
position adjustment can cause a short-radius curvature of the rudder
cable at extremes of pedal deflection. The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point.

As a counterexample rudder actuation system, I submit the Diamant. The
makers went to heroics to reduce rudder actuation friction, using push-
pull tubes in linear roller bearings with many ball bearing pivots and
a rather complicated pedal adjustment system. What they got was rudder
flutter, and they ended up having to incorporate a hydraulic shock
absorber in order to apply damping to the system. So they started with
a complicated system and ended up having to make it more complicated
yet before it was fully functional. Think of all the things they could
have done with their energy had they just used a standard cable system
and moved on.

*I wonder how many were killed by this design, giving many unexplained spins into
the ground from higher altitude.

My guess is few to none. Here in the US, crash investigations of light
aircraft, especially those of gliders, do tend to be less systematic
than those of larger aircraft. However, in my experience crash
investigators are fully competent at recognizing the signatures of
wear and fatigue failures in cable-actuated control systems. Where
such signatures are found, they are usually announced prominently in
the accident synopsis.

Thanks, Bob K.

"The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point. "

Apparently this is the solution SH used on at least some of their
sailplanes.
Upon recent inspection on my Mini Nimbus there is no wear after many
years.

On May 16, 2:10*am, GC wrote:
Also, with great respect to Bob K and his experience, I'm not convinced
airflow will sufficiently straighten a rudder with a broken cable. *Even
in the best case there will be residual deflection where the airflow
balances the other spring. *This could be quite large.

Isn't that something that can be easily (and safely) verified? In many
gliders, rudder cables are accessible in the cockpit. Take a high tow,
grab one cable and pull it. This simulates broken spring or broken
cable on one side. If the glider becomes uncontrollable, just release
the cable.

Bart